1. Field of the Invention
The present invention relates to a light modulator. More particularly, it relates to a light modulator utilizing surface plasmon, a light source and a display apparatus using the light modulator, and a method for driving the light modulator.
2. Description of the Related Art
As a conventional display apparatus, especially, a field sequential display apparatus, there is disclosed an apparatus using a color filter disk (eg., xe2x80x9cColor Liquid Crystal Displayxe2x80x9d written and edited by Shunsuke Kobayashi: published in Dec. 14, 1990, P. 117) With this apparatus, a color filter disc colored in red, green, and blue, i.e., three primary colors of light is placed in front of a monochrome cathode ray tube, and rotates in synchronism with display to enable color display. Similarly, color display is also possible even by placing the color filter disc in front of a white light source, and combining a black shutter (black-and-white shutter type display device) therewith.
Further, as an apparatus of another system, on pages 120 and 121 of the aforesaid document xe2x80x9cColor Liquid Crystal Displayxe2x80x9d, there is shown an apparatus for performing field sequential color display by combining two high-speed liquid crystal display devices referred to as xcfx80 cells, and a total of three sheet polarizers and color sheet polarizers in front of a monochrome CRT(cathode ray tube).
Further, there is also shown the technology using a CRT, LED (light emitting diode), or cold cathode fluorescent tube as a backlight, and using a liquid crystal display as a black shutter on pages 122 and 123 in the document xe2x80x9cColor Liquid Crystal Displayxe2x80x9d. With this apparatus, backlights of respective three primary colors are prepared so as to alternately flash. One example thereof is shown as a field sequential full color LCD in xe2x80x9cMonthly Published Displayxe2x80x9d, the July issue, pp. 11-16, (1998). In this example, the cold cathode fluorescence tube backlight which is illumination light of commonly used liquid crystal display apparatus is temporally switched among red, green, and blue.
In recent years, there are proposed a light modulator utilizing an electromagnetic wave supported by the interface between a metal and an insulator (a dielectric material) referred to as a surface plasmon wave (SPW), a display apparatus which permits color display by utilizing this light modulator, and a light source thereof as a display apparatus whereby color display is implemented without using the foregoing color filter and color light source. That is, conductors such as metals can be defined as a gas of electrons in electrostatic equilibrium inside a continuum of positive fixed charges. It can be considered as a xe2x80x9ccondensedxe2x80x9d electron plasma with electron density approximately equal to 1023 electrons per cm3. There exists a longitudinal wave referred to as a surface plasma oscillation in addition to a volume plasma oscillation which is a normal plasma oscillation. The electric field due to the surface plasma oscillation has a periodic wave form in a direction parallel to the metal surface, while having a form of evanescent wave which evanesces exponentially in a direction perpendicular to the metal surface. Plasmons are quanta associated with the plasma oscillation (collective wave excitation of a conductive electronic gas) in the metal. Because of high electron density, quantum effects dominate. The surface plasmon waves can be optically excited by resonant coupling. The condition for resonance is strongly dependent on the refractive indices and thickness of the media near the metal-insulator interface. The intensity of the light wave can be modulated by coupling the light wave with the surface plasmon wave. Generally, if coupling between the surface plasmon wave and the light wave is strong, the attenuation of the emitted light wave is strong, and if coupling is weak, there occurs almost no attenuation of the emitted light wave.
Attenuated total reflection (ATR) effect has been utilized to optically excite surface plasmon waves through a high-index prism. Light, traveling in free-space, is sent toward the metal-insulator interface through the prism with an angle larger than the critical angle, producing an evanescent wave field which may overlap the surface plasmon wave field. If the propagation constant Kev of the evanescent wave is in harmony with the propagation constant Ksp of the surface plasmon, the surface plasmon resonance is excited on the metal surface. Two configurations are mainly used for optically exciting the surface plasmon wave. The first is Otto""s ATR configuration. This Otto""s configuration is shown in FIG. 1A. In this Otto""s configuration, there exists a small air gap between a metal medium layer 101 stacked on a thick insulator 102 and a high-index prism 103. A surface plasmon wave 105 is optically excited by the incident light. Further, the second configuration used to optically excite surface plasmon waves is Kretschmann""s modified ATR configuration as shown in FIG. 1B. In this configuration, a thin metallic foil 101 is inserted between the prism 103 and the insulator 102. Surface plasmon waves 105 are also optically excited by absorbed light which will not be reflected light 106. This configuration is more practical since there is no air gap. It is noted that the high-index prism 103 for generating the evanescent wave may be a diffraction grating with a period smaller than the wavelength of the incident light, or other optical components.
Here, when a prism is used as an optical component, the propagation constant (wave number) Kev of the evanescent wave is represented by the following equation (1):                                                                         K                ev                            =                                                                    n                    ⁡                                          (                      ω                      )                                                        ·                                                            K                      0                                        ⁡                                          (                      ω                      )                                                        ·                  sin                                ⁢                                  xe2x80x83                                ⁢                θ                                                                                        =                                                                    n                    ⁡                                          (                      ω                      )                                                        ·                                      ω                    /                    c                                    ·                  sin                                ⁢                                  xe2x80x83                                ⁢                θ                                                                                        =                                                                    n                    ⁡                                          (                      λ                      )                                                        ·                  2                                ⁢                                                      π                    /                    λ                                    ·                  sin                                ⁢                                  xe2x80x83                                ⁢                θ                                                                        (        1        )            
where c is the speed of light in vacuum, xcfx89 is the angular frequency, xcex is the wavelength, n(xcfx89) and n(xcex) are the refractive indices of the prism in the case of an angular frequency xcfx89 and a wavelength xcex, respectively, K0(xcfx89) is the wave number in the case of an angular frequency xcfx89 in vacuum, and xcex8 is the incident angle of light with respect to the underside of the prism. Therefore, the wave number of the evanescent waves can be harmonized with the propagation constant of the-metal surface plasmon by adjusting the refractive index n(xcfx89) or n(xcex) of the prism and the incident angle xcex8 of light.
On the other hand, the propagation constant Ksp of the surface plasmon is given by the following equation 2, where the angular frequency of the surface plasmon is xcfx89, and the dielectric indices of the metal and the dielectric indices of the low-index medium in contact with the metal are respectively xcex5m and xcex50,                                                                         k                sp                            =                                                ω                  c                                ·                                                                                                                              ϵ                          m                                                ⁡                                                  (                          ω                          )                                                                    ·                                              ϵ                        0                                                                                                                                      ϵ                          m                                                ⁡                                                  (                          ω                          )                                                                    +                                              ϵ                        0                                                                                                                                                                    =                                                                    2                    ⁢                    π                                    λ                                ·                                                                                                                              ϵ                          m                                                ⁡                                                  (                          λ                          )                                                                    ·                                              ϵ                        0                                                                                                                                      ϵ                          m                                                ⁡                                                  (                          λ                          )                                                                    +                                              ϵ                        0                                                                                                                                                    (        2        )            
where the xcex5m(xcfx89) and xcex50(xcfx89) are the dielectric indices of the metal in the case of the angular frequency xcfx89 and the wavelength xcex, respectively. Here, since the xcex5m is a complex number, the propagation constant Ksp is also a complex number. The evanescent waves generated by using a prism when Kev=Ksp generates the surface plasmon. In order to strongly excite the metal surface plasmon, the metal surface plasmon itself must be a wave with a long life. That is, it is required that the imaginary part of the propagation constant Ksp is small, and the attenuation associated with propagation is small.
The imaginary part of the propagation constant Ksp is approximatively solved, assuming that respective complex-numbers are Ksp=Kspxe2x80x2+iKspxe2x80x3, and xcex5m=xcex5mxe2x80x2+ixcex5mxe2x80x3 (the xcex5, to be precise, depends on the angular frequency or the wavelength), yielding the following expression 3:                               xe2x80x83                ⁢                                                                              k                  sp                  xe2x80x3                                ≈                                  xe2x80x83                                ⁢                                                                            ω                      c                                        ·                                                                  (                                                                                                                                                                              ϵ                                  m                                                                ⁡                                                                  (                                  ω                                  )                                                                                            xe2x80x2                                                        ·                                                          ϵ                              0                                                                                                                                                                                                            ϵ                                  m                                                                ⁡                                                                  (                                  ω                                  )                                                                                            xe2x80x2                                                        +                                                          ϵ                              0                                                                                                      )                                                                    3                        ⁢                        R                                                                              xc3x97                                                                                                              ϵ                          m                                                ⁡                                                  (                          ω                          )                                                                    xe2x80x3                                                              2                      ⁢                                                                        (                                                                                                                    ϵ                                m                                                            ⁡                                                              (                                ω                                )                                                                                      xe2x80x2                                                    )                                                2                                                                                                                                                                    ≈                                  xe2x80x83                                ⁢                                                                                                    2                        ⁢                        π                                            λ                                        ·                                                                  (                                                                                                                                                                              ϵ                                  m                                                                ⁡                                                                  (                                  λ                                  )                                                                                            xe2x80x2                                                        ·                                                          ϵ                              0                                                                                                                                                                                                            ϵ                                  m                                                                ⁡                                                                  (                                  λ                                  )                                                                                            xe2x80x2                                                        +                                                          ϵ                              0                                                                                                      )                                                                    3                        ⁢                        R                                                                              xc3x97                                                                                                              ϵ                          m                                                ⁡                                                  (                          λ                          )                                                                    xe2x80x3                                                              2                      ⁢                                                                        (                                                                                                                    ϵ                                m                                                            ⁡                                                              (                                λ                                )                                                                                      xe2x80x2                                                    )                                                2                                                                                                                                                    (        3        )            
Therefore, the factor which decides the intensity of the metal surface plasmon is xcex5mxe2x80x3/(xcex5mxe2x80x2)2 (the xcex5, to be precise, depends on the angular frequency or the wavelength), and the metal to excite the metal surface plasmon is desirably a metal whose value of |xcex5mxe2x80x3/(xcex5mxe2x80x2)2 | is small. Specifically, silver (Ag), gold (Au), copper (Cu), aluminum (Al) and the like are usable.
There are proposed a liquid crystal display device (U.S. Pat. No. 5,451,980), and a projector (U.S. Pat. No. 5,570,139), each of which selects a wavelength based on an electric field using a material whose refractive index varies with the application of electric field such as a liquid crystal as a low-index dielectric to perform display utilizing surface plasmon. The example of the publication of the device in the academic meeting is shown in xe2x80x9cAppl. Phys. Lett.xe2x80x9d, U.S., 1995, vol. 67, the 19th number, pp. 2759 to 2761. In this reference, as shown in FIG. 2A, there is illustrated a device whereby the absorption wavelength is made variable, and the wavelength region of the reflected light is electrically changed using a liquid crystal. Further, FIG. 2B shows the measurements (a broken curve) and the calculated results (solid curve) of the characteristics of the reflected light intensity with respect to wavelengths when the voltage value in the aforesaid device is varied. In the device, a 60xc2x0 SF6 glass prism is used as a prism 103, and a 50-nm silver thin film is evaporated thereon as a thin metal film 101. A 50-nm MgF2 layer is then evaporated at a 50xc2x0 oblique angle onto the silver film as an alignment layer 108. A substrate 110 is so configured that the same alignment layer 108 is obliquely evaporated on an ITO film which is a transparent electrode formed on a glass substrate. A 4-xcexcm cell gap is ensured by spacers 109, and then filled with BL009 manufactured by Merck KGaA as a liquid crystal 107. As shown in FIG. 2A, a white light is incident on the device through a sheet polarizer as a p- and linearly polarized light, and a voltage is applied thereto to determine the dependence of the reflected light on the wavelength. The results are shown in FIG. 2B. At a voltage of 0 V, there is an absorption maximum in the vicinity of 640 nm. The absorption maximum shifts towards the lower wavelengths with the application of a voltage. At 10V, it is at 560 nm, and at 30 V, it is at 450 nm. The measured results are in good agreement with the calculated results.
Further, as another technology, there is a technology utilizing re-radiation of the absorbed light. As the example thereof, a description will be given to the technology shown in xe2x80x9cSID 97 DIGESTxe2x80x9d U.S., 1997, pp. 63-66. FIG. 3A is a cross sectional view of a device for obtaining transmitted light in a specific wavelength range by implementing the conventional Kretschmann method in a symmetric structure. FIG. 3B is a diagram showing the calculated results of the transmitted light intensity with respect to the wavelength when the refractive index of the central medium is changed in the device. This device is considered the same as the one obtained by removing the substrate 110 side, and providing a structure identical with the upper side structure on the lower side in symmetric relation in the device having the structure shown in FIG. 2A. However, the film thickness of the electro-optical material 111 of the central part is set much thinner as compared with the device of FIG. 2A. This symmetric structure and the very thin central electro-optical material 111 enable the coupling of the surface plasmon wave generated at the interface on the incident side to the surface plasmon wave at the next interface with the metal to generate another surface plasmon wave on the outgoing side. This surface plasmon wave re-radiates the light with the same wavelength. In this manner, it is possible to re-radiate the absorbed light. FIG. 3B shows theoretical calculated results when the anisotropic refractive index dn of the central material is changed from 0 to 0.2, and 0.5 in this device. When the dn is 0, the device radiates the light of a wavelength of 450 nm, at 0.2 and 0.5, 530-nm light and 650-nm light are radiated, respectively. Since surface plasmon is a surface effect, the film thickness of the central material is set to be very thin, or about one wavelength in order to effect such re-radiation. When a liquid crystal material is used as the central material, it is considered that the response speed can be about two orders of magnitude faster than the response speed of a conventional liquid crystal device because of the thinness of the film thickness.
Furthermore, as still other technology of the reference, the technology as shown in FIG. 4 shows an example of the configuration of a direct-view type liquid crystal display apparatus for performing a field sequential display utilizing surface plasmon. Here, one device of FIG. 3A is used as a device 100. The light from a line source 112 is incident through a cylindrical lens 113 on the device 100, and one color of the three primary colors of light is selected for every time period. The light is applied onto the whole surface of a liquid crystal panel 115 by a reflector 114 having a stepped surface to permit field sequential display. Further, in patent publications or other references, there are proposed other liquid crystal display devices or projectors utilizing them. As the technology described in U.S. Pat. No. 5,570,139, an example of a light source for a liquid crystal display device utilizing surface plasmon as shown in FIG. 5 will be described. White light is applied from the upper right to be sequentially incident on a plurality of (three) unit devices 100A, 100B, and 100C. Thus, the outgoing light therefrom is established itself as a light source. At respective unit devices 100A, 100B, and 100C, light beams of specific wavelength ranges, i.e., blue, green, and red light beams are individually absorbed to obtain respective colors of yellow, magenta, and cyan. This cycle is repeated in three devices to obtain a light of a prescribed color.
On the other hand, as an example in which surface plasmon is utilized for a projector, there is a technology shown in xe2x80x9cSPIExe2x80x9d, vol. 3019, pp. 35-40 (1997). FIG. 6 is a cross sectional view of an example of the projector utilizing surface plasmon according to the technology. A central surface plasmon device 100D has almost the same structure as the one shown in FIG. 5, except that the prisms 103 of FIG. 5 are integrated into one unit. There are placed a lamp 116, a reflector 117, a relay lens 118, an integrator 119, and a sheet polarizer 120 on the incident side. On the other hand, there is placed a reflection type liquid crystal display apparatus or the like, not shown, which performs monochrome modulation through a projection lens 121 to obtain an image on the outgoing side, thus performing image display. The light from the lamp is collected in one direction by the reflector, and then brought close to a parallel beam in a narrow region by the relay lens and the integrator. The light beam is then aligned into either polarized light by the sheet polarizer, and selection of color and image display are performed at the surface plasmon device. Finally, the image is projected through the projection lens. The projection can be accomplished based on the field sequential display.
Further, in Japanese Laid-Open Patent Publication No. Hei 5-313108, there is disclosed a light modulator in which a metal-insulator interface is formed adjacent to a planar wave guide for carrying a light wave. With the light modulator, a high frequency voltage applied to the interface causes the insulator to resonate, generating a surface plasmon wave on the interface. The resulting wave is coupled to a gradually vanishing light wave in the wave guide, thus changing the intensity of the light wave.
The foregoing prior-art light modulator, especially, the light modulator utilizing surface plasmon, and a display apparatus utilizing the same, encounter the following problems. The first problem lies in that the structure is complicated. This is attributable to the fact that, in the prior art, three unit devices each having a wavelength variable by an electric field are required for simultaneously obtaining light beams of three primary colors of specific wavelength ranges. The second problem lies in that loss of light is high. The reason for this is that only the reflected light from which light at surface plasmon has been absorbed, or the re-radiated light of the absorbed light is utilized. Therefore, the whole of light which has not been used results in a loss. For example, with the technology of FIG. 4, the period during which one color produced by the field sequential display is displayed, other 2 colors of the three primary colors are not utilized at all. The third problem lies in that there exists no light modulator capable of ensuring a division both temporally and spatially.
It is therefore an object of the present invention to provide a light modulator with a simplified structure. It is another object of the present invention to provide a light modulator whereby the loss of light is minimized. It is a still further object of the present invention to provide a light modulator capable of ensuring division both temporally and spatially. It is yet further object of the present invention to provide a display apparatus using the aforementioned light modulator, and a display method thereof.
The present invention relates to a light modulator utilizing surface plasmon generated at the interface between a thin metal film and an electro-optical material. Then, the present invention is characterized by including two unit devices, and a mirror, the two unit devices, each comprising: a pair of prisms individually provided with thin metal films at their respective undersides, and the thin metal films being oppositely disposed, and an electro-optical material sandwiched between the oppositely disposed thin metal films, wherein the two unit devices are disposed in parallel to each other such that respective one surfaces of the one prisms of the unit devices are in contact with each other so as to ensure the arrangement of the thin metal films in parallel relation to each other, and the mirror is disposed such that the mirror side thereof faces a direction in parallel to the thin metal films, and extends along the top of the prism of one unit device on the side thereof not in contact with another unit device. As the electro-optical material, a liquid crystal material is used. Alternatively, an air gap can be adopted in place of the electro-optical material. The thickness of the air gap may also be changed by a piezo material provided between the prisms constituting the unit device. Further, it is possible to adopt a diffraction grating in place of the prism. Furthermore, a light source can be configured, or a liquid crystal display apparatus or a liquid crystal projector can be configured, by utilizing the foregoing light modulator.
With the light modulator according to the present invention, all of the incident light can be utilized in the final outgoing light with no loss by making both of the transmitted light due to absorption and re-radiation generated by the unit device and the reflected light into the outgoing light, the light incident on the next unit device, or the light incident on the mirror. Further, the color of light can be spatially divided. Still further, it can also be divided temporally by changing the wavelength by a voltage.